As disclosed in JP H9-181771 A, in an Ethernet (registered trademark) based communication system, an Ethernet switch utilized as a relay apparatus detects an abnormality occurrence in a received frame and discards the abnormal frame.
In the above system, when the Ethernet switch discards the abnormal frame, a transmission source of the frame cannot recognize the discarding of frame. Thus, the transmission source of the frame cannot take measures, such as retransmission of the frame against the frame discarding.